Equine sarcoids are the most common form of skin neoplasia in horses, with an owner reported frequency of 5.8% (Ireland et al, 2013). Bovine papillomavirus (BPV) types 1 and 2 are causally associated with the development and pathogenesis of equine sarcoids (Taylor and Haldorson, 2013). Sarcoids occur commonly in areas where biting flies congregate, including the medial aspects of the thighs, the ventral abdomen and axillae and around the eyes and ears. They can also occur on the distal limbs or trunk, particularly if there has been a previous wound. This has led to the speculation that the biting fly (Stomoxys calcitrans) may be a vector for transmission of disease. Research has shown that stable flies exposed to both equine sarcoid and bovine papilloma tissue resulted in a significant viral load within the fly, indicating that the flies may act as a vector for sarcoid transmission (Haspeslagh et al, 2018).
There are six different types of sarcoids based on the gross appearance: occult, verrucose, nodular, fibroblastic, mixed and malignant sarcoids (Knottenbelt, 2019). Depending on the location, the size and extent of skin ulceration, sarcoids can cause significant morbidity to equids and can be expensive and difficult to treat, resulting in significant expense to horse owners.
A multitude of over 40 treatments have been described to treat equine sarcoids, broadly categorised into surgery, chemotherapy, immunotherapy, gene therapy, radiotherapy, photodynamic and other therapies (Knottenbelt, 2019). With no one size fits all treatment for all lesion types available, the treating veterinary surgeon must take into consideration multiple factors, including the size and number of lesions present, the duration of clinical signs, if any previous treatment has been performed and costs of treatment, along with owner and horse compliance.
Surgical removal
Traditionally, surgical removal of sarcoids has had a very poor success rate with local recurrence rates of 72% being reported (McConahy et al, 1994). The main reason for such a poor outcome with traditional sharp surgical removal is that poor surgical margins will ‘seed’ tumour cells into the surgical site which can lead to instant regrowth of the lesion, often in a more aggressive manner than the original presentation. More recently a technique of ‘smart’ surgery has been described (Knottenbelt 2015), the aim of which is to reduce the likelihood of seeding tumour cells into the wound bed during surgical removal of the tumour; this type of surgery also has the additional benefit of achieving first intention healing of the surgical wound.
It is not possible to visually identify the margin between tumour and normal tissue. Analysis of tissue from surgical margins of sarcoids with polymerase chain reaction identified the presence of bovine papillomavirus in 33% of lesions with a margin of 16mm (Martens et al, 2001). However, subsequent research failed to identify a correlation between the presence of papillomavirus in the surgical margin and recurrence of sarcoids (Taylor et al, 2014), making the prediction of recurrence of equine sarcoids following resection very challenging.
Cryosurgery
Cryosurgery has been used for decades for the treatment of equine sarcoids. It involves the controlled use of freezing temperatures to destroy undesirable tissue, while causing minimal damage to surrounding tissues. Cell destruction is best accomplished with rapid freezing and slow thawing of tissue using three ‘freeze-thaw’ cycles, and is most commonly accomplished using liquid nitrogen. The procedure can be repeated at intervals several weeks apart, until the area is tumour-free, although it is somewhat difficult to determine the depth of the tissue being frozen, so treating bulky lesions can be more challenging. The use of thermocouples to precisely measure the temperature of the frozen tissue and the depth of the freeze, may allow more accurate treatment (Fretz and Barber, 1980). The procedure can result in significant sloughing of frozen tissue which can take 2–3 weeks following treatment. There can also be significant damage to surrounding structure including blood vessels, nerves, tendons and ligaments if treatment is too aggressive. Additionally, white hairs frequently develop at the site of treatment (Hanson et al, 1997). There are several publications documenting the use of cryosurgery to treat equine sarcoids, although direct comparison of the studies is difficult because of the use of several adjunctive treatments, differing lengths of follow up and the lack of histopathological examination of the sarcoids. Published rates of recurrence vary from 5–85% (Compston et al, 2016)
Laser surgery
Laser surgery is now frequently used as a first line treatment for a variety of sarcoids, owing to its relatively low cost and the ease of use along with wide availability. The most common types of lasers used in clinical practice are CO2 and diode lasers. CO2 and diode lasers vary in the wavelength of light that they produce, CO2 laser light has a relatively long wavelength of around 10 600 nm and a low penetrative depth that results in a high precision cut, with a minimal zone of thermal damage of -0.1mm. This allows the direct suturing of incisions made with a CO2 laser (Carreira and Azevedo, 2016). Diode lasers produce light with a lower wave-length of 800–1100 nm, this results in a thousand-fold increase in absorption or penetrative depth and a much wider zone of thermal damage (Compston et al, 2016).
Laser surgery should be performed in a suitable environment, owing to health and safety concerns associated with the use of lasers. Appropriate training and the use of adequate personnel protective equipment is mandatory. The room where surgery is to be performed should have lockable doors, to prevent people from entering while surgery is taking place, along with blinds to cover any windows. Additional signage indicating that laser surgery is taking place should be clearly displayed outside the entrance to the room.
During surgery, staff should be aware that lasers generate heat and extra care is needed if using any flammable drapes around the surgical site. Surgical spirit should not be used to prepare sites before surgery, owing to the risk of significant burns to the patient. Surgery can be performed under general anaesthesia or in most instances can also be carried out using standing sedation and local or regional analgesia (Figure 1). Given the extension of tumour cells into the surrounding tissues a surgical margin of 1.5–2 cm should be used to remove the sarcoid, with close attention paid to avoiding contact of the wound bed with the surface of the sarcoid (Figure 2).
In most instances, following sarcoid removal with a laser, the wounds are left to heal by secondary intention. If a diode laser has been used and there has been significant thermal damage to the tissues, then the wound appearance often gets worse before it gets better, as the surface of the burn sloughs. Healing of wounds can be prolonged, especially if the lesions that have been removed were large. Wound management normally consists of saltwater bathing and the use of oral non-steroidal anti-inflammatory drugs. Careful consultation with owners before surgery is essential to ensure they are fully aware of what to expect following surgery.
If large lesions are removed from the axilla or high in the groin (both common sites for sarcoids), there can be restriction of movement as wounds heal by contraction and as the resulting scar tissue is weaker than normal tissue, the scar can tear It is therefore important to ensure adequate movement during healing. Additionally stretching exercises can help soften scar tissue during healing and reduce the risk of secondary injury.
Removal of sarcoids with a diode laser may have the additional benefit that the wider zone of thermal damage that occurs compared to a CO2 laser may reduce the likelihood or tumour recurrence owing to a reduction in tumour cells within the zone of necrosis. Additionally, laser removal may limit ‘seeding’ of tumour cells from the lesion surface or release from the cut ‘roots’.
Recurrence rates for sarcoids following removal with a CO2 laser range from 6–38% (Carstanjen et al, 1997; Kemp-Symonds et al, 2008). Higher rates of recurrence at the surgical site (65%) were observed to occur in animals <6 years of age (Carstanjen et al 1997). Reports on the success of using a diode laser to remove histologically confirmed sarcoids had an 83% non-recurrence rate at the site of tumour removal and a 73% non-recurrence rate for any other site following surgery (Compston et al, 2016). Recurrence rates in cases which had received prior treatment increased up to 50%. Additionally, it was noted in this study that verrucose sarcoids had an increased risk of recurrence following laser surgery and it was hypothesised that this was because of the histological characteristics of verrucose lesions extending further into the dermal layer than other types of lesions.
In some locations laser surgery in not appropriate, particularly with periocular lesions when excessive scarring could cause functional problems with the eyelids. Caution should also be sued in location where there may be important anatomical structures such as nerves, blood vessels and synovial structures.
It has been generally accepted that the greater the overall number of lesions is a negative prognostic indicator for successful treatment (Haspelagh et al, 2016). This is likely to be a result of several factors including the overall ‘tumour load’, along with the patient's immune response or susceptibility to developing lesions, along with leaving tumour tissue in the wound at the end of the surgical procedure. Sarcoids are most likely to recur during the first 6 months following surgery.
It is recommended that all skin masses that are surgically removed are submitted for histopathological analysis to determine the presence of sarcoid and check for the presence of any cells in the wound margins, which may indicate that recurrence of the lesion is more likely. If sarcoids recur following surgical removal, then surgery can be repeated, or adjunctive chemotherapeutic options may be considered useful.
Adjunctive chemotherapy
In some instances, chemotherapeutic agents can be used at the time of surgery to reduce the risk of local recurrence. Chemotherapeutic agents carry additional health and safety risks to the operator and horse owner and should be used with appropriate personal protective equipment (Smith et al, 2018). The use of cisplatin beads has been reported in conjunction with laser surgery (Hewes and Sullins, 2006). Laser surgery was used to debulk the lesions before implantating the bioabsorbable beads around the wound base and margin with a success rate of 85%. The beads slowly release cisplatin into the area following implantation, with the aim of preventing tumour recurrence; unfortunately the beads are relatively expensive and must be imported, requiring a Special Import Certificate from the Veterinary Medicines Directorate. The use of cisplatin injections as an oil emulsion at the time of surgical removal, or following healing of the surgical wound, has also been evaluated and appears to be safe without compromising wound healing (Theon et al, 1999). Intralesional mitomycin C has also been reported in the literature to treat sarcoids (Malalana et al, 2010), and intralesional injections can be used as an adjunctive therapy following surgery (Figure 3).
More recently the use of electrochemotherapy has been reported at the time of surgery, with impressive non-recurrence rates of 97.9% (Tamzali et al, 2012). Electrochemotherapy uses electrical field pulses to increase cell membrane permeability to hydrophilic drugs such as cisplatin. This results in a significant increase in the intracellular concentration, giving significant therapeutic benefit. The use of laser surgery to ‘debulk’ lesions before performing electrochemotherapy, reduced the number of treatments required to result in a successful outcome (Figure 4a–c). Treatment must be carried out under general anaesthesia as the electrical stimulation can cause significant muscular contractions. Treatments are repeated 2-weekly intervals until the tumour has regressed. General anaesthesia will result in additional treatment costs and some additional risk to the patient. However, Platt et al (2017) showed that horses undergoing repeated episodes of general anaesthesia have a better recovery quality.
Conclusions
Sarcoids are the most common of skin neoplasia in the horse, with a wide variety of treatments options available. Surgical removal of sarcoids using laser surgery can provide a cost-effective way of treating many lesions. There are numerous surgical approaches to treatment, although there is a lack of good quality evidence-based research comparing the different treatment options available.
KEY POINTS
- Equine sarcoids are the most common form of skin neoplasia in the horse, with a reported prevalance of 5.8%.
- Sarcoids can occur in locations that can be difficult to treat and can cause significant pain and morbidity to the patient.
- There are many treatment options available to the treating veterinary surgeon and treatment protocols vary depending on the lesion location, extent, previous treatments, and financial implications.
- Surgical treatment can be a relatively simple, cost effective and safe way of dealing with selected sarcoids.